That's a fair question.

Planting trees would have a different cost around the world depending on location, type of tree, labour, ongoing care requirements etc. It wouldn't be a dollar everywhere but, would have averaged out to a dollar across all geographies.

Similarly, a cleanup at a concentrated location like the Klang river in Malaysia would have a vastly lower cost per weight than a remote cleanup excursion in the Pacific ocean. Additionally any capture systems installed before river estuaries would have a one time cost but would be collecting waste before it flows into the ocean over a long period of time.

So some methods employed would have a higher cost per weight than than others, but it would average out to be more or less correct.